Lithographic masks (reticles) are used in semiconductor fabrication to transfer structure patterns, which are formed thereon, of a ready-patterned mask layer onto a substrate, in particular a semiconductor substrate or a wafer, on a reduced scale with the aid of lithographic exposure. Regions of the substrate that correspond to the transmissive mask regions are exposed and can be patterned, etched or exposed to ion implantations and other processing steps selectively with respect to unexposed surface regions. The mask may be a transmission mask or, as is customary in particular in the range of very short wavelengths of just a few nanometers, a reflection mask. Different transmission or reflection at mask structures and the spaces between them lead to the optical transfer of the mask pattern to the substrate, which is to be exposed.
A lithographic mask is exposed a number of times per wafer, in order for in each case different regions of the wafer to be exposed. Furthermore, in mass production a multiplicity of semiconductor wafers are exposed using a single mask.
Over the course of time, impurity particles and other impurities may be deposited on the mask, having an adverse effect on the quality of the optical imaging and leading to defects on the structure which is imaged on a reduced scale onto the exposed semiconductor wafers. In particular, in the region of the focal plane of the mask layer, the mask structures and spaces between the mask structures of the mask layer, defining the position of the unexposed and exposed regions on the semiconductor wafer, are distorted if, for example, a foreign particle enters a space between adjacent mask structures where it leads to local shadowing of a corresponding region on the semiconductor wafer, which as a result is not exposed as intended. This produces defects in the semiconductor circuits, which are fabricated.
To avoid such defects, it is true that it is possible for a mask to be cleaned at relatively long intervals, but this operation is complex and increases production costs.
To prevent the deposition of impurities or impurity particles in the focal plane of the mask layer, a flexible or fixed membrane, known as a pellicle, is often arranged at a certain height above the outer side of the mask layer. This membrane is secured along the edge of the mask with the aid of a membrane holder or pellicle frame. The membrane transmits light and does not impede the optical imaging, but also keeps impurity particles away from the outer side of the mask.
However, the membrane holder is provided with openings at the edge, in order to allow pressure compensation in the event of differing air pressure conditions, for example during transport in air, and to avoid pressure-induced damage. Therefore, even when a pellicle is fitted, contact with the clean-room air is possible via the openings in the pellicle frame, and over the course of time this leads to contamination to the patterned mask layer. These contaminations, for example caused by ammonium sulfide, amines, sulfur oxides or impurity particles, are present in trace amounts even in clean-room air and, once they have penetrated between the pellicle and the mask layer, cannot be removed again without complex cleaning. On the other hand, completely closing up the pellicle frame would no longer allow pressure compensation.
U.S. Pat. No. 5,928,827 A and German Patent No. 38 42 481 A1 have disclosed lithographic masks whose mask structures are covered with a continuously dense protective layer. These continuously dense protective layers keep impurity particles away from the mask structures more reliably. However, the production of continuously dense protective layers involves more time and labor, since their surface, which is remote from the mask layer, has to be planarized at considerable cost in order not to adversely affect the quality of the optical imaging. In particular, on account of the greater layer thickness of the continuously dense protective layers compared to pellicle membranes, much lower roughness and deviations in the surface of the protective layer from the position parallel to the mask substrate are permissible in the case of the continuously dense protective layers.